The present invention relates to a light-modulating liquid crystalline composition and also to a liquid crystalline display device.
Liquid crystals have been used in a wide variety of electro-optic display applications. These include, in particular, electro-optic light-modulating applications which require compact, energy-efficient, voltage-controlled light modulator, such as watch, calculator, and laptop computer displays. Their popularity is due to the conveniently thin, flat shape and the very low power required. In these devices, a thin layer of liquid crystal (usually nematic) is sandwiched between parallel cell walls, which have been treated to control the alignment of the liquid crystal director. When a potential difference is applied to transparent electrodes on either side of the liquid crystal, the resulting electric field causes a realignment of the molecules and a change in the optical behavior of the layer.
The twisted nematic display is commonly used for digital watches and other small displays. The surfaces of the cell are treated so that, in the absence of an electric field, the local liquid crystalline directors are all coplanar, but twist through 90xc2x0. Light entering the cell is polarized parallel to the directors at the top surface. The polarization follows the twist in the directors and the light passes through the polarizer at the bottom. The light is then reflected by a mirror and reverses its path to emerge at the top surface. This area appears bright. In an area in which the electric field is turned on, the directors align with the field throughout most of the sample. Now the beam""s polarization is not rotated by the liquid crystal directors and the light is absorbed by the second polarizer. This area appears dark.
Cholesteric liquid crystals have been used in reflective displays without a backlight and without a polarizer. Cholesteric liquid crystals generally exhibit three states. In the first, the cholesteric axis is oriented normal to the plane of the display. This is known as the planar or Grandjean state. The planar state will reflect light by the Bragg effect. The reflected wavelength is related to the pitch and the average refractive index of the liquid crystalline material by the relationship xcex=na*P/2, where P is the 360 degree pitch length of the cholesteric. Thus, the planar state may appear colored and reflective or, if the pitch is in the infra-red, transparent. The second state is achieved by the application of an electric field sufficient to disrupt the planar state into a disordered, focal conic state. Depending on the nature of the cholesteric material and the cholesteric pitch, the focal conic state may be weakly or strongly light scattering. At higher voltages, the helical pitch is completely unwound and the cholesteric molecules become oriented perpendicular to the plane of the display cell. This is known as the homeotropic state, which is transparent. In pure cholesteric materials, the planar state is stable, the homeotropic state is unstable and the focal conic state is metastable, taking from seconds to hours to revert to the planar state upon removal of an electric field. By stabilizing the appropriate cholesteric state with a polymer network, the focal conic-planar transition time may be greatly reduced. Alternatively, the focal conic state can be stabilized such that it reverts to the planar state only if first switched to the homeotropic state, allowing bistable displays to be made. Examples of these two types of operation are as follows. One type of cholesteric has a first state having a color-reflecting planar configuration. A low-frequency voltage pulse disrupts the alignment of the cholesteric into a light-scattering focal-conic state, which persists after the end of the pulse. A short, higher voltage pulse restores the initial configuration. Another type of cholesteric liquid crystal has a light transparent state and a light scattering focal-conic state. This type of cholesteric is used in the reverse-mode cell display.
Hikmet, U.S. Pat. No. 5,188,760, discloses a polymer stabilized liquid crystal display cell. The cell comprises a low-molecular-weight liquid crystalline material and a polymerizable liquid crystalline material. A polymer is formed from monomers having mesogenic groups positioned between two reactive groups. Unfortunately, the cell requires a higher switching voltage than is desired in certain applications.
It is therefore an object of the present invention to provide an improved liquid crystal display device.
Other objects and advantages will become apparent from the following disclosure.
The present invention relates to a display device having light-modulating composition comprising a low molecular weight liquid crystalline material dispersed in a polymer including a non-mesogenic crosslinking monomer reacted with a mesogenic monomer comprising a mesogenic group, a spacer, and one or more reactive functionality. Preferably, the reactive functionality is a vinyl group.
In one embodiment of the present invention, the mesogenic monomer has only one reactive functionality and an alkyl spacer is positioned between the mesogenic group and the reactive functionality. The resulting comb polymer is lightly crosslinked. The compositions of the present invention surprisingly exhibit improved light modulating properties including lower driving voltage and/or faster switching speed.
A more thorough disclosure of the present invention is presented in the detailed description which follows and the accompanying drawing.